1. Field of the Invention
This invention relates to positioners or actuators for controlling the accessing and tracking of magnetic heads in rotating magnetic memory systems, and particularly to high performance moving head actuators using a pivotal arm movement.
2. History of the Prior Art
In the relatively short history of magnetic disc file recording, the great majority of earlier positioners for selecting individual tracks for signal recording and reproduction have used radially movable mechanisms. Examples of this type of positioner are provided by U.S. Pat. Nos. 3,614,333, 3,637,928, 3,643,036 and 4,005,493. Although mechanical and hydraulic devices have sometimes been used as shown by these patents, most radial positioners now employ what are known as voice coil drives for translation of electrical signals into linear motion and precise positioning. Energization of a relatively large cylindrical coil coupled to the arm and head structure causes movement of a magnetically biased armature to a chosen track position. Constant improvements in recording technology and demand for increased storage capacity have imposed often conflicting requirements on positioner designs. A high force or torque relative to inertia must be generated in order to keep the motion time interval between track locations low. With track densities of 300 tracks per inch and sometimes substantially more, mechanical deflection can be a significant problem and structures must be devised that have proper rigidity. With high track densities and disc speeds, high servo bandwidths are required.
To meet these and other requirements, voice coil actuator types of systems have generally utilized a large voice coil, and have provided roller bearing mounts for the carriage. Because of the linear movement of the structure, its entire mass contributes to the inertia that is to be overcome, and the mass cannot be substantially reduced. Furthermore, bearing wear introduces inevitable variations in performance under the continuous, high speed operation to which these units are subjected.
There has been another trend in disc file configurations, arising also from the need for increased storage capacity. In this relatively newer trend, the discs are incorporated as fixed media within an hermetic enclosure, and the air within the enclosure is constantly filtered. Thus when the enclosure is assembled and closed and the system is thereafter operated, dust and other impurities are much less apt to affect performance. Contaminants become very significant at high recording densities and track densities, because the heads must be flown at a very close spacing from the surface of the disc.
Consideration of these problems had led to the introduction of a fixed media fully enclosed type of system in which what is termed a swinging arm mechanism is used as the head positioner. Such a system is described in an article entitled "Design Of A Swinging Arm Actuator For A Disc File," by J. S. Heath, pp. 389-397 of the IBM Journal of Research and Development for July 1976. The referenced article describes how a pivotal movement of the arm and the geometry of the head structure can be utilized for a high performance system, despite the changes in circumferential position of the head with different track locations. In the system described by Heath, the positioner arm is pivoted through a selected angle about a pivot axis which is intermediate the length of the arm. With the head mount and heads being coupled to one terminus of the arm, a voice coil structure is coupled to the arm on the opposite side of the pivot axis. The coil armature is interior to the coil, which has a rectangular cross-section and which does not provide a uniform magnetic interaction throughout the arc of pivotal movement. Furthermore, with the rectangular section of coil, there is a rather ineffective coupling between the magnetic field of the coil and the interior armature. Moreover, this entire structure is mounted within the enclosure and is therefore not easily accessible for service or replacement of the operative components. Because the mass of the coil and the supporting arm are at a radial distance from the pivot axis, the inertia of the entire structure is substantial, and therefore greater power is required for low motion intervals as the system is operated. All heat generated during energization of the actuator must be dissipated within the enclosure, which is an undesirable feature. The system is a successful one, but it would be highly desirable to eliminate or minimize some of the mentioned problems, and to substantially decrease the average access time by providing a more efficient system.